DraftProposal - Do not cite without author’s permission
Sun-Powered Laser Beaming from Space for Electricity on Earth
A Draft Proposal to DARPA by Eric & Marty Hoffert, Versatility Energy, South Orange, NJ 07079
ABSTRACT: Space solar power (SSP) offers the opportunity of breakthroughs in large-scale power generation and
highly flexible power distribution. To validate the vision of SSP, a research project is proposed here to demonstrate
basic capabilities of space solar power, including a demonstration of long-range wireless power transmission from
geosynchronous orbit (GEO) to the surface of the earth using high-energy lasers. JAXA, the Japanese space agency,
last year announced plans to launch by 2010 a satellite that will unfurl a large solar array and beam 100 kilowatts of
microwave or laser power to a receiving station on Earth (Gibbs, 2006). However, because diffractive beam spreading
requires large antennas at microwave frequencies, it would be virtually impossible to launch a microwave beamer
large enough for efficient space-to-Earth power transfer without expensive multiple launches and in-space assembly.
Limitations due to beam divergence from microwave transmission with its companion large-scale transmitters and
receivers can be overcome using the laser-based system outlined here. Advantages of a laser experiment include
demonstration of continuous (24/7) electric power transfer from orbit and reception on Earth orders of magnitude
greater than anything that has been done historically. With laser beaming, “on the shelf” ultralight solar panels, and
other early launch opportunities (such as exploitation of NASA Geo QuickRide or suitable alternatives), this milestone
appears achievable in a three to five year time frame; perhaps even before the announced JAXA SSP tests. This
proposal is focused on the identification of key technology issues in the real world, setting the stage for commercial
and military space solar power to provide electricity on-demand where and when needed -- a high priority for national
security in a world in which currently inexpensive liquid and gaseous hydrocarbons will be increasingly scarce and
more costly, and where powerful new methods are needed for flexible power delivery and distribution. Furthermore,
expensive and cumbersome methods of transporting large amounts of fuel could be replaced with lighter weight and
simpler power receiving equipment, with the potential to revolutionize localized power generation and distribution
capabilities.
GOAL & MOTIVATION: Research is proposed to laser beam solar power collected by space-based ultralight
photovoltaics (PV) to surface arrays tuned to laser bandgaps (Landis, 1994). A specific goal is GEO orbit-insertion of a
to-be-developed experimental payload in a single launch in five years or less. Strategic applications of space solar power
include global-scale electric baseload supply and electricity-on-demand (Lior, 2001; Hyde et al., 2006). Satellites in
GEO (geostationary orbit 36,000 km above the equator) are widely employed to beam information worldwide because
they appear stationary relative to Earth’s surface. GEO powersats exploit (almost) continuously available solar flux S o 
1.37 kW/m2 from sun-tracking collectors for order-of-magnitude more DC power per unit area than the long-term
average of highly-variable & low areal power density solar and wind at the surface (Hoffert, 2006a). Continuous power
from SSP is critical because energy storage is the cost-pacer limiting market penetration of terrestrial renewables (Love
et al., 2003). [That Denmark has the world’s peak renewable electricity market penetration (20%) is possible only
because it’s windpower is backed-up by pumped storage from Norway’s 100% hydroelectric system.] The advantage of
higher solar flux in space is partly offset by launch costs and costs of wireless power transmission (WPT). But even with
presently high PV costs (> $5000/peak kW e) electricity from orbit can have comparable costs to terrestrial-solar for
launch costs close to the present 15,000 $/kg for Russian & Chinese launches with geosynchronous transfer capacity
(Hoffert, 2006a; Futron, 2002).
WHY LASERS? SPS needs to project through the atmosphere, which is transparent in visible and microwave
bandwidths, but mostly opaque elsewhere. Space-to-Earth WPT can be as simple as space mirrors bouncing sunlight to
surface receivers (Ehricke, 1979). But geometric optics implies that the subtended sun angle projects an over 400 km
diameter spot size from a GEO onto Earth’s surface. Too big. Better, but still too big for the near-term, are PV-driven
cloud-penetrating microwave beamers, as in the NASA-DOE reference design of the ‘70s (Koomanoff and Bloomquist,
1998) and the NASA “Fresh Look Study”(Mankins, 1997). Microwave beaming is limited by diffractive spreading
proportional to (wavelength)/(transmitter aperture) driving SSP toward big space antennas (~ 1 km) and bigger surface
rectennas (~10 km). Microwave WPT could work for global-scale electric base load but require high initial capital costs
for “first power.” And small scales won’t work for microwaves. One solution is laser beams with 100,000 times smaller
than microwave wavelengths and negligible spreading (Hoffert et al., 2004). Lasers are employed, for example, for lunar
ranging by bouncing beams back from corner reflectors 400,000 km distant left by Apollo astronauts on the Moon
(Dickey et al., 1994). Diode lasers are commercially available with 50% efficiencies at low power (Dickinson, 2002).
Comparably efficient lasers with continuous power of hundreds of kW are under development for DARPA by our
partner, General Atomics. Siting receivers in deserts can address potentially intervening cloud scattering, with
-2subsequent routing though terrestrial power grids, for electric utility
applications, and multi-receiver “hopping” for power “on demand”
(Hyde et al., 2003). Supplying the electricity demand curve of Europe
by the laser SSP system depicted in the inset (PV arrays in GEO, lasers
beaming to PV receivers in the Sahara & electricity transmitting to
Europe via HVDC lines) was assessed by Geuder et al. (2004), who
also compared it to purely terrestrial PV with pumped hydro storage.
With economies of scale they find laser SSP cost-effective at terawatt
power levels (1TW = 1012 W). Carbon-emission-free primary power of
10-30 TW could be needed by midcentury to stabilize climate against
catastrophic global warming with continued economic growth (Hoffert,
2006b). Today, global electricity generation is 1.9 TW e requiring 5.7
TW primary power to generate, projected to triple by 2050. Our
proposed experiment can test laser SSP technology near-term, fostering
early implementation of this revolutionary global-scale emission-free
electricity source.
POWER, MASS & COST: To capture the most solar energy at lowest cost we want the largest solar collector feasible
in a single launch and reliable space-based components with highest specific power (power per unit mass, P/M). The
largest single-vehicle-launched structure so far in space may be Signals Intelligence (SIGINT) "Trumpet," with a
reported 100-meter dish (Couvault, 1997). NRO doesn’t release such information, but reportedly in the unclassified
literature Titan 4s launched three Trumpets since 1994 in Molniya-type elliptic orbits. It seems safe therefore to specify a
thin-film PV collector diameter D  100 m. Also assumed are efficiencies in the chain from sunlight in GEO to DC on
Earth: PV, space  (PV DC power output)/(solar photon power incident)  10%, laser  (laser coherent photon power
out)/(space PV power in)  10%, trans  (laser photon power captured at surface)/(laser coherent photon power
transmitted)  80%, PV, surface  (DC power to grid or direct consumers)/(laser photon power captured at surface)  80%
(assumes PV arrays are tuned to the laser bandgap energy). Some intermediate efficiencies are absorbed in these
numbers. Note our near-term technology conservative assumptions: 10% efficient amorphous silicon (a-Si) films (not the
20% or more attainable down the road) and 10% efficient high power diode laser (not the 50% under development).
Electric power available at the surface is then
Psurface = PV, space laser trans PV, surface (/4) D2 So  70 kWe (direct current)
The example illustrates that a single-vehicle launch should be able to orbit a laser SSP payload package capable of
illuminating 70 kilowatts continuously e.g., electrifying a remote outpost or small village where and when needed.
Moreover, recent breakthroughs indicate high specific power in the range of 1 to 5 kW e/kg including support structures
may be available for deployable thin-film PV arrays. The
inset shows an ESA-DLR ultra-light 20-meter spacequalified thin film PV array with supporting booms, with
400 m2 of CP1/a-Si:H thin film cells of mass 32 kg
providing 50 kW of solar power deployable from the small
“suitcase” at center (P/M ~ 1.6 kWe/kg, including support
structure). The record for amorphous silicon on 6 m CP1
films is ~ 4 kWe/kg (Wyrsch et al., 2006). With inflatablerigidizable structures gross power densities in the range of
1 to 5 kWe/kg appears feasible. A 10% efficient array 100
m in diameter in GEO outputting P PV ~ 1000 kWe (1 MWe)
with (P/M) PV ~ 2 kWe/kg including support structure
would mass out to 500 kg. Adding 200 kg for the laser and
300 kg for “balance-of-system” (power conditioning,
attitude controls, telemetry, etc.) give ~ 1000 kg (one
metric tonne) payload. At $15,000/kg launch costs to
GEO, the price to insert this package in orbit would be $15
million. Even if launch costs were $30 million, perhaps 1/3 of the program costs, a budget of order $100 million is
reasonable for the five-year project.
-3The funding outlined here is a very modest level of investment when compared with competing large-scale power
generation alternatives such as nuclear fusion. In addition, the payback for the approach proposed here could be very
substantial, as foundation technology to enable Megawatt and Gigawatt class power generation and distribution would be
proven and prepared for scalable deployment.
APPLICATIONS: The research proposed here is targeted to produce a successful system for power generation and
transmission systems using high powered lasers. Once such a milestone can be achieved in the development of the core
enabling technologies for WPT and SSP, there are a large number of unique and important applications which can be
supported, such as flexible power distribution delivered to: (a) isolated or advanced positions; (b) airplanes, or high
altitude airships; (c) large-scale ships at sea; (d) satellites requiring new power sources (i.e., to address battery depletion
or eclipse constraints); or (e) offshore bodies such as islands and man-made platforms. Additional capabilities may
include support for on-demand high-scale power generation in remote areas; movement of objects from LEO to GEO;
high-energy electrolysis to produce hydrogen from water for use in fuel cells in vehicles, aircraft, etc.; and support of
broader goals for energy security and energy independence. Certain applications such as power delivery to high-altitude
airships or ships at sea requires complementary tracking to ensure power is delivered accurately to these moving objects.
RESEARCH PLAN: We envision this project as a typical $300-500 k/year DARPA start-up expanding over five years
to a full-scale multi-phase space experiment as critical milestones are accomplished. Early on in a Phase I program, we
plan a high technical level multidisciplinary Workshop organized by the PIs in which technical issues are identified and
analyzed. For example, the ultralight, or “gossamer,” orbiting structures envisaged here are subject to solar radiation
pressure affecting attitude control and station keeping; the implications of these effects need to be better understood.
Likewise, the appropriate scale at which meaningful power beaming tests become feasible must be determined; and the
impact of weather on beam propagation etc must be reviewed. There also needs to be an evaluation of the deployment of
high powered lasers in space where issues such as cooling are very different and more challenging as there is only
radiation to dissipate easily. Ancillary topics such as eye safety and other environmental issues need to be covered as
well. We need to understand these examples of issues and many others as we venture into uncharted territory, despite the
availability of selected parts of the system “on the shelf.”
Phase 1 will also include design of a comprehensive and detailed research program. This work will be followed in Phase
II by a preliminary design of a series of experiments, including lab tests, and possible mountain-to-mountain tests of
laser beaming efficiency. In parallel, we will be building a team of consultants, industrial collaborators and mission
analysts, focused quantitatively on a successful laser SSP experiment as outlined above. Phase III would be focused on
the successful demonstration of laser power beaming from space to earth. Versatility’s role will be systems integration
and multi-disciplinary science and engineering leadership in cooperation with the DARPA Program manager. Technical
and administrative details of this project, while eminently doable, are challenging, and will require a more extensive
exposition than space permits here. However, our basic idea, methods and cost estimates are laid out here to stimulate
discussion and set the stage for the next, we hope more serious, stage. Upon an indication of interest with respect to
this executive summary, a more detailed project plan and formal proposal will be submitted with respect to DARPA’s
established “Seedling” program for funding innovative new projects.
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